Development and application of the coarseness exponent as a new index for evaluating the coarseness of marine sand and gravel

The overall coarseness of marine sand and gravel (MSG) is a critical parameter for quality assessment, ore-body delineation and suitability evaluation. Current MSG exploration adopts the Udden-Wentworth scale for grain-size classification, whereas there is no dedicated coarseness index matched to this system. The fineness modulus ( M x ) derived from the American Society for Testing Materials sieve series is widely applied for coarseness evaluation. This study developed a new index, the coarseness exponent ( C e ), based on the Udden-Wentworth scale. The C e value is 0.25 greater than the corresponding M x value, and MSG is classified into five grades according to C e : ultra coarse, coarse, medium, fine and ultra fine. Measured data from 111 sandy samples validated the capability of C e to characterize the overall coarseness of MSG ( R 2 = 0.99, RMSE = 0.03). The practical performance of C e and M x was compared in MSG grading and ore-body delineation. The results show generally consistent classification and delineation outcomes for the two indices. Furthermore, C e allows valid grading of ultra-coarse MSG that cannot be defined using M x . Adopting C e in MSG exploration unifies the sand-mud boundary at 0.063 mm and avoids the need for two independent grain-size analyses. Consequently, C e exhibits promising prospects for MSG quality grading, ore-body delineation and exploitation-suitability assessment.